/* @(#)s_atan.c 1.3 95/01/18 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 *
 */

/* atan(x)
 * Method
 *   1. Reduce x to positive by atan(x) = -atan(-x).
 *   2. According to the integer k=4t+0.25 chopped, t=x, the argument
 *      is further reduced to one of the following intervals and the
 *      arctangent of t is evaluated by the corresponding formula:
 *
 *      [0,7/16]      atan(x) = t-t^3*(a1+t^2*(a2+...(a10+t^2*a11)...)
 *      [7/16,11/16]  atan(x) = atan(1/2) + atan( (t-0.5)/(1+t/2) )
 *      [11/16.19/16] atan(x) = atan( 1 ) + atan( (t-1)/(1+t) )
 *      [19/16,39/16] atan(x) = atan(3/2) + atan( (t-1.5)/(1+1.5t) )
 *      [39/16,INF]   atan(x) = atan(INF) + atan( -1/t )
 *
 * Constants:
 * The hexadecimal values are the intended ones for the following 
 * constants. The decimal values may be used, provided that the 
 * compiler will convert from decimal to binary accurately enough 
 * to produce the hexadecimal values shown.
 */

package kotlin.math.fdlibm

private val atanhi = doubleArrayOf(
    4.63647609000806093515e-01, /* atan(0.5)hi 0x3FDDAC67, 0x0561BB4F */
    7.85398163397448278999e-01, /* atan(1.0)hi 0x3FE921FB, 0x54442D18 */
    9.82793723247329054082e-01, /* atan(1.5)hi 0x3FEF730B, 0xD281F69B */
    1.57079632679489655800e+00, /* atan(inf)hi 0x3FF921FB, 0x54442D18 */
)

private val atanlo = doubleArrayOf(
    2.26987774529616870924e-17, /* atan(0.5)lo 0x3C7A2B7F, 0x222F65E2 */
    3.06161699786838301793e-17, /* atan(1.0)lo 0x3C81A626, 0x33145C07 */
    1.39033110312309984516e-17, /* atan(1.5)lo 0x3C700788, 0x7AF0CBBD */
    6.12323399573676603587e-17, /* atan(inf)lo 0x3C91A626, 0x33145C07 */
)

private val aT = doubleArrayOf(
    3.33333333333329318027e-01, /* 0x3FD55555, 0x5555550D */
    -1.99999999998764832476e-01, /* 0xBFC99999, 0x9998EBC4 */
    1.42857142725034663711e-01, /* 0x3FC24924, 0x920083FF */
    -1.11111104054623557880e-01, /* 0xBFBC71C6, 0xFE231671 */
    9.09088713343650656196e-02, /* 0x3FB745CD, 0xC54C206E */
    -7.69187620504482999495e-02, /* 0xBFB3B0F2, 0xAF749A6D */
    6.66107313738753120669e-02, /* 0x3FB10D66, 0xA0D03D51 */
    -5.83357013379057348645e-02, /* 0xBFADDE2D, 0x52DEFD9A */
    4.97687799461593236017e-02, /* 0x3FA97B4B, 0x24760DEB */
    -3.65315727442169155270e-02, /* 0xBFA2B444, 0x2C6A6C2F */
    1.62858201153657823623e-02, /* 0x3F90AD3A, 0xE322DA11 */
)


private const val one = 1.0
private const val huge = 1.0e300

internal fun atan(_x: Double): Double {
    var x: Double = _x
    var w: Double
    var s1: Double
    var s2: Double
    var z: Double
    var ix: Int
    var hx: Int
    var id: Int

    hx = __HI(x)
    ix = hx and 0x7fffffff
    if (ix >= 0x44100000) {    /* if |x| >= 2^66 */
        if (ix > 0x7ff00000 ||
            (ix == 0x7ff00000 && (__LO(x) != 0))
        )
            return x + x        /* NaN */
        if (hx > 0) return atanhi[3] + atanlo[3]
        else return -atanhi[3] - atanlo[3]
    }
    if (ix < 0x3fdc0000) {    /* |x| < 0.4375 */
        if (ix < 0x3e200000) {    /* |x| < 2^-29 */
            if (huge + x > one) return x    /* raise inexact */
        }
        id = -1
    } else {
        x = fabs(x)
        if (ix < 0x3ff30000) {        /* |x| < 1.1875 */
            if (ix < 0x3fe60000) {    /* 7/16 <=|x|<11/16 */
                id = 0; x = (2.0 * x - one) / (2.0 + x)
            } else {            /* 11/16<=|x|< 19/16 */
                id = 1; x = (x - one) / (x + one)
            }
        } else {
            if (ix < 0x40038000) {    /* |x| < 2.4375 */
                id = 2; x = (x - 1.5) / (one + 1.5 * x)
            } else {            /* 2.4375 <= |x| < 2^66 */
                id = 3; x = -1.0 / x
            }
        }
    }
    /* end of argument reduction */
    z = x * x
    w = z * z
    /* break sum from i=0 to 10 aT[i]z**(i+1) into odd and even poly */
    s1 = z * (aT[0] + w * (aT[2] + w * (aT[4] + w * (aT[6] + w * (aT[8] + w * aT[10])))))
    s2 = w * (aT[1] + w * (aT[3] + w * (aT[5] + w * (aT[7] + w * aT[9]))))
    if (id < 0) return x - x * (s1 + s2)
    else {
        z = atanhi[id] - ((x * (s1 + s2) - atanlo[id]) - x)
        return if (hx < 0) -z else z
    }
}
